An electrical power converter is a device for processing electrical power from one form into another form that meets the requirements of an electrical system. Electrical power converters commonly are used to change alternating-current (AC) power to direct current (DC) power or vice versa, or to change one DC voltage level to another. A power converter that changes DC power to AC power is commonly referred to as an "inverter." A variety of circuit topologies are used as the basis for power converters, including the well-known topologies referred to as buck, boost, buck-boost, push-pull, full bridge, half-bridge and forward converters.
In many power converters, a switching regulator regulates the output signal of the converter by varying the amount of time that electrical energy is coupled through a power switch from the converter's input to a power transformer within the power converter, the output of the power transformer being filtered to produce an output voltage. The regulation process commonly is performed by a pulse width modulator that is responsive to the output voltage of the power converter. The pulse width modulator controls the amount of time the power switch is ON (i.e., electrical energy, in the form of voltage and current are enabled to couple through the switch), thereby determining the pulse width of the current and voltage pulses coupled from the input of the converter. The pulse width (sometimes called length) of the input voltage pulses (as well as the shape of input current pulses) in turn affects the characteristics of the output voltage.
An example of a typical prior art high frequency switching mode power inverter 500 is shown in FIG. 1. The illustrated inverter produces a rectangular AC output waveform V.sub.inv with sharp rising and falling edges. Power inverter 500 has two stages. The first stage is a power converter 503 which converts a low DC input voltage V.sub.in from a battery 502 into an output DC high voltage, V.sub.o across an output capacitor 528. This DC voltage V.sub.o is chopped to produce an AC output voltage V.sub.INV by inverting the DC voltage V.sub.o during alternate voltage half-cycles of V.sub.INV by a second stage full bridge inverter 504. Bridge inverter 504 is typically switched at a 50 Hz or 60 Hz rate to produce an output AC voltage V.sub.inv at output terminals 505. Those skilled in the art will recognize that power converter 503 is a push-pull converter which utilizes the following conventional components: primary switches 506 and 508, a power transformer 510 having primary windings 512 and 514 and a secondary winding 516, output rectifiers 518, 520, 522 and 524, output choke 526, and output capacitor 528. Switches 506 and 508 are controlled by a conventional pulse width modulator circuit (not shown).
The characteristics of the output voltage waveform of a power inverter are often important to the performance of the load device and for the power inverter to meet standard specifications. For example, it is generally important to regulate the RMS value of the output power, voltage, and/or current. As another example, many simple off-line uninterruptable power supplies ("UPS's"), which draw DC input power from a battery, are designed to produce a rectangular or trapezoidal AC output voltage, instead of the customary sinusoidal AC output. UPS's often are also required to have waveforms with rising and falling edges of less than 10 volts/microsecond.
The reasons for these requirements are well-known and relate to efficient battery utilization and control of electrical noise. More specifically, for a simple off-line UPS, the shape of the output current waveform is nearly the same as the shape of the input current waveform. Both current waveforms are related to the shape of the output voltage waveform. If the output voltage of the off-line UPS is sinusoidal, for example, both the output current and input (battery) current waveforms consist of relatively sharp peaks near the center of each voltage half cycle, with the value of the input current peak being much greater than the value of the output current peak. By contrast, if the output voltage of the off-line LIPS has a rectangular or trapezoidal waveform, the output current and battery current waveforms approximate these voltage waveforms.
Given the same output power being generated by a battery powered UPS, the peak input current that is required in order to generate a sinusoidal output voltage is much higher than the peak input current in the case of a trapezoidal output voltage. As is well known, this higher peak current for a sinusoidal output voltage places greater stress on the battery side of the power converter and results in the usable percentage of battery capacity being lower for a sinusoidal output off-line UPS than for a rectangular or trapezoidal output UPS. For this reason, in simple off-line UPS designs, rectangular or trapezoidal wave outputs are generally preferred.
However, when a UPS having a rectangular or trapezoidal AC output with a sharp rising edge (slope&gt;&gt;10 V/.mu.s) is used to drive a typical load, its output current will have a high leading current spike at the beginning of each voltage half cycle. This high current spike will generate noise, which will sometimes interfere with the normal operation of the device being powered, such as a computer or computer monitor. For this reason, it is desirable for UPS's to have an output voltage rising edge slope of less than 10 V/.mu.s. For similar reasons, it may be important to keep the falling edge slope below a certain value. Most simple types of UPSs have output voltage waveforms with sharp rising and falling edges.
Accordingly, there is a need for an electrical power inverter having a trapezoidal output voltage waveform and rising and/or falling edges that are kept below a certain slope value.